Thousands gathered to fill Foley Square, in downtown Manhattan, with song as they remembered Eric Garner, Mike Brown and other unarmed black men killed by police before fanning out to march on the city's streets Thursday night.

As the sounds of police sirens beeped intermittently and a helicopter buzzed above, protesters clapped their hands as they sang: "I still hear my brother crying, 'I can't breathe.' Now I'm in the struggle saying, 'I can't leave.' We're calling out the violence of racist police. We ain't gonna stop 'til our people are free."

Crowd organizers urged demonstrators in the green space near the New York Police Department's headquarters to be respectful of each other as a cordial tone blanketed the lively crowd on a second night of nationwide protests sparked by a Staten Island grand jury decision Monday not to indict the white police officer for the chokehold death of Garner.

The 43-year-old asthmatic died in July. His final words were multiple please of "I can't breathe" as New York Police Officer Nick Palanteo had him in a chokehold. Michael Brown died in August and the officer who shot him to death on a Ferguson street, Darren Wilson, was not indicted last week, prompting a string of violent and peaceful protests in many U.S. cities.

Wednesday night's demonstrations in New York, Oakland, San Francisco and Washington, D.C., caused disruptions and delays as protesters took over streets, highways and bridges, but they were largely peaceful.

At the Foley Square rally, where authorities estimated more than 5,000 had gathered, Constance Malcolm, the mother of Ramarley Graham, an unarmed black 19-year-old who was killed by plainclothes police in 2012 after being chased into his Bronx apartment, called on the crowd to remember Garner, to remember Brown, to remember her son, too.

Several reporters on the ground noted that authorities warned thousands of protesters may be heading to the Brooklyn Bridge, where a sit-in stopped traffic Wednesday night. Via its Twitter account, the city's Office of Emergency Management warned residents of "extensive traffic delays" on the bridge due to "protest activity," encouraging commuters to consider alternate routes.